It is known to use optical communications media to carry optically modulated data over great distances in communications networks. However, known optical media (for example optical fibres) are known to exhibit characteristics which limit the effective distance over which optical signals can be transmitted without requiring detection and regeneration in order to avoid irretrievable corruption of the modulated data. Such characteristics include both chromatic dispersion effects and polarisation mode dispersion (PMD) effects. Both of these effects effectively impose practical limits on the length of optical transmission media from which the optical signals can be recovered using known technology and at reasonable cost without suffering intolerable loss of data (whether through lack of reliability in the data recovered, or through total loss of the signal).
As a result of these effects, it is often necessary in an optical communications network to detect and to regenerate the modulated data at intermediate points between source and destination nodes, particularly where the source and destination nodes are a large distance apart. The optical receiving and re-transmission apparatus required at such intermediate points may be complex, costly, and bulky and require ongoing expense arising from powering costs and maintenance costs. The overall cost of this equipment is ultimately borne by the end-customers making use of such communications networks.
Two known forms of signal detection for such optical systems are coherent detection and direct detection.
In systems employing coherent detection, the transmitted signals may be transmitted over a broad band of frequencies. At the receiver, the signal—having been subjected to the degrading effects of dispersion as noted above—is combined with a local oscillator signal so as to extract the original signal. This offsets the received signal to a lower frequency band. Detection is then performed on the signal at the lower frequency band. However, the degrading effects of dispersion in the transmission medium which most affect coherent receivers arise as a result of the width of the frequency band rather than the individual frequencies themselves, and so this offset to lower frequencies, whilst retaining the information content of the signal, does not materially mitigate the effects of dispersion since the lower-frequency signal still requires at least the same width of band as the originally transmitted signal. The phase distortion damage is done during transmission, all coherent mixing down does, is to frequency shift that distorted information.
Known direct detection transmission systems are also limited by optical dispersion. Conventionally, this dispersion is compensated optically: high levels of dispersion cannot be compensated electronically after conversion from the optical domain by means of known methods using a single photo-detector to convert from optical domain to electrical domain.
It is therefore clearly desirable to provide improved methods and apparatus which will extend the effective reach of such optical transmission systems so as to reduce the requirement for such regeneration apparatus and thereby mitigate these disadvantages.